.container {
  height: 100px;
  border: 1px solid #c3c3c3;
  display: flex;
}
.one , .two {
  flex-basis: 50px;
}

.one {
  background-color: coral;
}

.two {
  background-color: lightskyblue;
  animation: basis 5s infinite;
}

@keyframes basis {
  50% {
    flex-basis: 200px;
  }
}